High-temperature Oxidation Conditions Research Articles

Microscopic insights into the ammonia-methanol blended combustion at high temperatures

Ammonia and methanol have a broad application prospect in the future as green renewable liquid fuels. Methanol has high reactivity and good solubility with ammonia, which is conducive to solving the dilemma of ammonia combustion. However, there are few studies on ammonia-methanol blended combustion, and the effect of blended combustion on the reaction pathways needs to be further investigated. Therefore, this study used the reaction molecular dynamics simulations to reveal the role of methanol on the ammonia combustion and emission process at the microscopic level, focusing on the effects of different fuel blending ratios and oxygen concentration in the air under stoichiometric conditions. The variation of oxidation pathways from ammonia to NO and methanol to CO under different conditions was analyzed in detail. It was found that the methanol accelerated ammonia combustion mainly through enhancing OH production, but methanol and its intermediates inhibited the initial oxidation of ammonia. When the fuel blending ratio was 0.75, the system had lower fuel-type NO production alongside a higher ammonia consumption rate under stoichiometric conditions. The crucial role of NH in the HNO formation at high-temperature ammonia oxidation conditions was revealed. The pathway of HNO + OH → NO + H2O dominated the conversion of HNO to NO in this study. Increasing the methanol content promoted the conversion of NH to HNO and the pyrolysis of HNO while inhibiting the reaction between HNO and O2. In addition, the finding indicated that the high O2 level in air can enhance the reactions between ammonia and OH radicals but inhibit the pyrolysis of CH2OH/CH3O significantly.

Regeneration of copper catalysts mediated by molybdenum-based oxides

Cu catalysts, known for their unparalleled catalytic capabilities due to their unique electronic structure, have faced inherent challenges in maintaining long-term effectiveness under harsh hydrogenation conditions. Here, we demonstrate a molybdenum-mediated redispersion behavior of Cu under high-temperature oxidation conditions. The oxidized Cu nanoparticles with rich metal-support interfaces tend to dissolve into the MoO3 support upon heating to 600 °C, which facilitates the subsequent regeneration in a reducing atmosphere. A similar redispersion phenomenon is observed for Cu nanoparticles supported on ZnO-modified MoO3. The modification of ZnO significantly improves the performance of the Cu catalyst for CO2 hydrogenation to methanol, with the high activity being well maintained after four repeated oxidation-reduction cycles. In situ spectroscopic and theoretical analyses suggest that the interaction involved in the formation of the copper molybdate-like compound is the driving force for the redispersion of Cu. This method is applicable to various Mo-based oxide supports, offering a practical strategy for the regeneration of sintered Cu particles in hydrogenation applications.

Atomic origin of CO2-promoted oxidation dynamics of chromia-forming alloys

The development of atomic imperfections within oxide films from high-temperature oxidation of heat-resistant alloys significantly limits the self-protectiveness of the surface oxide, contributing to the failure of energy generating system components such as turbines, engines, and heat exchanges. Directly probing the dynamics of such atomic defects is challenging because of the extreme thermochemical conditions of high-temperature oxidation. Using environmental transmission electron microscopy observations, here we directly capture atomic-scale dynamics of vacancies in growing Cr2O3 film during high-temperature oxidation of NiCr alloy in CO2. Coordinated with theory modeling, we delineate the atomistic mechanisms associated with the effect of interstitial carbon derived from CO2 on promoting the formation, migration and clustering of atomic vacancies to result in the enhanced alloy oxidation. The identified oxidation mechanism can find broader applicability in utilizing the atmosphere to tune the formation and evolution of atomic-scale defects, thereby affecting the mass transport properties of the growing oxide film.

Improving the high-temperature oxidation resistance of CrN coating by gradient deposition considering internal stress effects

In this study, two different CrN coatings were deposited on the surface of Zr-4 via a multi-arc ion-plating method using gradient and non-gradient deposition processes. The effects of two deposition processes on the high-temperature oxidation resistance and internal stress of the coatings were investigated. The microstructure and phase structure of the coatings were characterized by scanning electron microscopy, X-ray diffraction (XRD), and energy dispersive spectrometry. The high-temperature oxidation resistance of the specimens was investigated by performing thermal shock experiments and simulated loss-of-coolant accident high-temperature steam oxidation experiments. The residual stresses of both coatings during deposition were determined by XRD analysis. In addition, the thermal stresses of both coatings during the high-temperature oxidation tests were modeled and calculated. Under long-term high-temperature oxidation conditions, the gradient-deposited CrN coating exhibited improved high-temperature oxidation resistance performance than that exhibited by non-gradient-deposited CrN coating. In addition, the residual stresses of gradient-deposited CrN coating during deposition and thermal stresses during high-temperature oxidation were significantly lower than those of the non-gradient-deposited CrN coating. The results obtained herein confirm the promising potential of gradient-deposited CrN coating as an accident-tolerant fuel cladding material.

Distinctive performance evolution of surface layer in TC4 alloy oxidized at high temperature: Softening or hardening?

The characteristics of metal surface layer at high-temperature significantly affect the service performance of TC4 titanium alloy components, so that the evolution mechanism of mechanical properties of metal surface layer under high-temperature oxidation conditions is an important scientific issue. In the present work, the mechanical behavior and the relevant mechanism of metal surface oxidized at the temperatures of 500 ℃, 700 ℃, and 900 ℃ for different times have been systematically investigated. The experimental results show that with the increase of temperature, the oxidation kinetics curve of the alloy changes from parabolic law to linear law, and the oxidation reaction changes from diffusion control to interfacial reaction control. Furthermore, as the temperature increases from 500 °C to 900 °C, the feature of oxide scale changes from the single-layer structure to the multi-layer alternating structure, and the thickness of oxide scales and depth of alpha case increased with increasing temperature. The surface oxide scale is mainly composed of low valent oxides such as Ti3O and Ti6O during low-temperature oxidation as well as short-term high temperature oxidation. After long-term high temperature oxidation, the oxide scale is mainly composed of high valent oxides such as TiO2, Al2O3, and TiVO4. Simultaneously, significant changes occur in the composition, microstructure and properties of metal surface layer. In general, the microstructure and mechanical properties of the surface-layer metal are strongly dependent upon the competitive influence of the outward diffusion of Al and the inward diffusion of O. In the temperature range of 500–900ºC, when the temperature is relatively low, the softening effect caused by the outward diffusion of Al is stronger than the strengthening effect caused by the inward diffusion of O, and a surface softening effect thus occurs. On the contrary, when the temperature is high, the strengthening effect caused by the inward diffusion of O is stronger than the softening effect caused by the outward diffusion of Al, and a surface strengthening effect accordingly happens.

Structural characteristics, oxidation performance and failure mechanism of thermal barrier coatings fabricated by atmospheric plasma spraying and detonation gun spraying

In recent years, thermal barrier coatings (TBCs) are widely applied to the surface of gas-turbine engines to prolong the service life of high temperature components. The increase in raw material requirements and costs has led to the search for an alternative, cheaper ceramic based material to yttria stabilized zirconia (YSZ), which is the traditional TBC top coating material. In this study, CoNiCrAlY metallic bond coating powders were deposited on Ni-based superalloy substrates by the detonation gun (D-gun) process. Commercial YSZ and blast furnace slag (industrial waste) powders were deposited on the bond coating by atmospheric plasma spraying (APS) technique. TBCs were subjected to isothermal oxidation tests for 8, 24, 50, and 75 h at 800 °C. The microstructural properties of the TBCs were evaluated comparatively using advanced characterization techniques before and after oxidation tests. It has been observed that the coatings produced using BFS material have a potential to be used in different types of industries as a low-cost and environmental solution against high temperature oxidation conditions.

Investigations into Power Plant Alloys’ (Inconel 718) Oxidation Resistance by Compound Composite (Cr2O3 + YSZ) Coatings

As it increases the pressure and temperature of incoming steam and decreases CO2 emissions, oxidation is crucial for materials used in power plants to increase their efficiency. Compound composite (Cr2O3 + YSZ) coatings applied to Inconel 718 (EN8) substrates using the Atmosphere Plasma Spray technique are anticipated to increase structural resistance when subjected to high pressure and temperature oxidation conditions of service. The nickel-based superalloys EN8 and EN8/Yttria (8% Y2O3) Stabilized Zirconia (YSZ)/Cr2O3 were subjected to high-temperature oxidation tests in the open air at 1050 °C for approximately 12, 24, 48, and 100 h. EN8 is not appropriate for prolonged use at 1050 °C as can be seen from scanned electron microscope and energy dispersive X-ray spectroscopy analyses on isothermally oxidized samples. The findings demonstrated that the EN8 alloy exhibited more significant weight variations over 48 h at high temperatures because its chromia oxide scale was continuously smaller. With phase dispersion in the microstructure, coated EN8 exhibits a higher performance under more prolonged exposure than the EN8 alloy. Additionally, the synthesis of outer chromium oxide, YSZ, and Cr2O3 on a substrate at 1050 °C for 100 h improved the outstanding oxidation resistance while maintaining the integrity of the chromium oxide layer.

Migration of harmful elements in municipal sludge under high-temperature oxidation conditions

Migration of harmful elements in municipal sludge under high-temperature oxidation conditions

Isothermal oxidation and hot corrosion behavior of HVOF sprayed 80Ni-20Cr coatings at 750 °C

NiCr thermal spray coatings are widely used in many different industrial areas, especially in aviation, automotive, and thermal power plants, in order to obtain abrasion resistance. The determination of high-temperature properties is of great importance in terms of determining the limits to which the coating can operate without being damaged and its structural stability. In this study, 80Ni-20Cr alloy powder was deposited onto 316L stainless steel substrates using a high velocity oxygen fuel (HVOF) spraying technique, and the coatings' oxidation and corrosion behaviors at high temperatures were assessed. In this context, while isothermal oxidation tests were conducted at 750 °C for 5, 25, 50 and 75 h, the hot corrosion behavior of the coatings was investigated for 1, 3 and 5 h at 750 °C in an environment consisting of a 55 wt% V2O5–45 wt% Na2SO4 corrosive salt mixture. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to evaluate powder feedstock and as-deposited, oxidized, and corroded coating samples' phase formation and microstructural changes. In all cases, the coated samples showed good resistance to isothermal oxidation, with no damage such as flaking and peeling. No oxide formation was observed on the coating surfaces. The oxidation started from the coating and substrate interface and proceeded towards the coating surface. Following the hot corrosion experiments, it was observed that the 80Ni-20Cr coating's reaction with the vanadate-sodium sulfate melt resulted in the formation of the corrosion product Ni3V2O8 in all time periods. As a result, under high temperature oxidation and hot corrosion environmental conditions, 80Ni-20Cr coatings have been observed to retain their durability without major damage.

High-temperature oxidation of zirconium alloys with magnetron sputtered chromium coatings

Loss-of-coolant accidents (LOCAs) often lead to dire consequences. That’s why the global community are actively engaged in research aimed at creating accident tolerant fuels. One of the areas of such research is preserving zirconium alloys as the material for fuel-element claddings by creating corrosion-resistant coatings that would be in contact with the coolant. Chromium could be a promising material to be used in such protective coatings as chromium oxide (Cr2O3) serves as an effective barrier for oxygen both in normal operation and in case of LOCA. Chromium coating hinders oxygen diffusion into the metal substrate and thus prevents embrittlement and failure of the fuel-element cladding. This paper describes the results of research studies that have been carried out in recent years and that look at the resistance of magnetron sputtered chromium coatings to high-temperature oxidation in water vapour up to 1,500 oC. The paper demonstrates advantages and drawbacks of chromium coatings in hightemperature oxidation conditions. The focus is on understanding how regimes of magnetron sputtering influence the resistance of chromium coatings to high-temperature oxidation and how the structure and phase state of chromium coatings are related to their properties. The authors describe the optimal regimes of magnetron sputtering for obtaining dense coatings and examine the effect of the substrate temperature and the bias voltage on the structure and density of resulting coatings. High Power Impulse Magnetron Sputtering (HiPIMS) serves as an effective technique that helps enhance the density of coatings. The conclusion drawn is that in order to broaden the temperature range in which chromium coatings can effectively protect zirconium alloys from failure up to 1500 oC, there should be a diffusion barrier between the surface of the fuel-element cladding and the chromium coating. The authors would like to thank B. A. Kalin, supervisor of this project, who had passed away before this publication was made. The staff of the Laboratory of Ion-Plasma and Ion-Beam Machining of Materials, a part of Department No. 9 at the National Research Nuclear University MEPhI, devote this paper to his memory.

Investigation on Microstructures and High-Temperature Oxidation Resistance of Cr Coatings on Zircaloy-4 by Multi-Arc Ion Plating Technology.

Introducing the oxidation-resistant coating on Zr alloy is considered to be one of the potential solutions for accident-tolerant fuel (ATF) materials. In this study, pure Cr coatings were prepared on a Zircaloy-4 (Zry-4) alloy surface by multi-arc ion plating under different process parameters. The ability of Cr coating on Zry-4 alloy cladding to improve the oxidation resistance to prevent a loss-of-coolant accident (LOCA) was studied. The microstructure of Cr coating was analyzed using the EBSD technique, and the high-temperature steam oxidation was tested at 800, 1000 and 1200 °C. Compared with the original Zry-4 alloy, the samples with Cr coatings exhibited much better oxidation resistance under different high-temperature steam oxidation conditions. However, the Cr coating exhibited columnar grain, strong preferred orientation and (001) fiber texture. The columnar grain boundaries provided paths for the diffusion of oxygen atoms to the Zry-4 alloy matrix at high temperatures. The results showed that the oxidation film of Cr coating with relatively random grain orientation was compact and uniform and exhibited the best oxidation resistance at high temperatures.

Structure and properties of multilayer coatings based on CoCrAlY/Al2O3 obtained by detonation spraying

Ceramic coatings are used to protect against high-temperature oxidation of metal products. The efficiency of the technology of detonation spraying of two-layer coatings of aluminum oxide ( Al2O3 ) and a heat-resistant alloy (CoCrAlY) on the surface of a sample made of steel 12X18H10T was studied. A multi-chamber detonation unit was used in the work, providing high speed (up to 1000 m/s) and a 60–80% utilization rate of the sprayed material. We used an oxygen-neutral combustible mixture based on propane-butane. The coating material has a smooth gradient of properties from ceramic to sample substrate The complex of the obtained structural and physical-mechanical properties of the coating material makes it possible to predict its high performance under conditions of high-temperature oxidation.

Synthesis of self-healing NiAl-Al2O3 composite coatings by electrochemical way

Nickel aluminide diffusion coatings act like an aluminium reservoir to form protective alumina scales at high temperatures. However, interdiffusion phenomena play a critical role in the metallurgical integrity of these coatings. This work explores a new type of self-healing coating for high temperature oxidation applications. Micro-reservoirs consisting of an aluminium-rich intermetallic core (Al3Ni2) and an aluminium oxide shell are embedded in a nickel aluminide matrix (NiAl). This new type of coating has been synthesized by an electrochemical chemical route to trap the micro-reservoirs in an electrodeposited nickel matrix followed by aluminization via a slurry route. Under high temperature oxidation conditions, the micro-reservoirs allow to form a diffusion barrier and to release Al into the coating matrix. These two simultaneous phenomena make it possible to maintain a sufficient amount of aluminium and ensure the unique formation of α-alumina at the surface of the coatings, hence to possibly increase the lifespan of the aluminium diffusion coatings.

High-temperature oxidation wear performance and mechanism of Cr–Mo (W)–V hot-work die steel

Abstract Dry sliding tests for Cr–Mo (W)–V hot-work die steels were performed at 400–700 °C using a UMT-3 wear tester. The wear behavior was studied systematically to explore the general characteristics under high-temperature oxidation conditions. When a mild-to-severe oxidation wear transition occurred, the wear rate of the Cr–Mo (W)–V hot-work die steels could be divided into different stages. The wear rate in stage I remained low and increased continuously with increasing temperature in stage II. It decreased within a narrow temperature range in stage III and increased significantly at a high test temperature in stage IV. The microstructures of DM, H21, and H13 steels at 350 HV30 were observed in stage III, and the M 2C carbides were extensively analyzed. Furthermore, by examining the continuous heating transformation curves, the decrease in the wear rate in stage III was qualitatively confirmed, depending on the transformation of the M 2C carbides.

Protective Cr coatings with CrN/Cr multilayers for zirconium fuel claddings

Cr coatings with CrN/Cr multilayers with a layer thickness of 100, 250 and 750 nm were deposited by a multi-cathode magnetron sputtering on E110 zirconium alloy in Ar and Ar + N2 atmosphere. The mechanical properties of the as-deposited coatings were measured by scratch testing and tribometry. A comparative study of crack and oxidation resistance of the Cr-coated E110 alloy was performed under thermal cycling (1000 °С) and high-temperature (1200–1400 °С) steam oxidation conditions. The influence of the CrN/Cr multilayers and their thickness on the mechanical properties and oxidation resistance of the coated E110 alloy is discussed. It is found that the CrN/Cr multilayers can limit interdiffusion between the coating and E110 alloy at high temperatures only for a short period.

Migration of Harmful Elements in Municipal Sludge Under High Temperature Oxidation Conditions

Migration of Harmful Elements in Municipal Sludge Under High Temperature Oxidation Conditions

HIGH-TEMPERATURE OXIDATION AND EROSION RESISTANCE OF NI-BASED THERMALLY-SPRAYED COATINGS USED IN POWER GENERATION MACHINERY: A REVIEW

The life of the components operating under various high-temperature environments decreases due to the activation of different failure mechanisms. High-temperature oxidation and erosion are the two prominent mechanisms that lead to the degradation of materials, resulting in the premature failure of the components. This paper has emphasized the failure and performance analysis of nickel-based coatings formulated by using different thermal spraying techniques. Nickel-based coatings like Ni–Cr, Ni3Al, Alloy-718, NiCrAlY, NiCrBSi and Ni-based composite coatings showed excellent resistance against the high-temperature conditions. This study helps to select specific thermal spray techniques and coating composition against high-temperature erosion and oxidation conditions.

Self-healing behavior of Cr-coated Zr alloy cladding in high temperature steam oxidation process

Cr-coated Zr alloy claddings, prepared via physical vapor deposition (PVD), were used to study the damage evolution under a series of high-temperature steam oxidation conditions. Through-thickness cracks were initially produced within the Cr coat by internal pressure creep test. The Cr-coated samples with cracks were then oxidized at temperatures from 800 °C to 1200 °C in water steam environment. Microstructures of the samples before and after oxidation were characterized in comparison by scanning electronic microscopy, transmission electronic microscopy and energy dispersion spectrometry. The results showed that a large proportion of cracks would be self-healed when the temperature increased to 1000 °C. At the places where the cracks have survived, the corresponding Zr substrate regions were oxidized to form semi-spherical or fan-shaped ZrO2. However, the maximum depth of ZrO2 under the unhealed cracks was smaller than that of the uncoated claddings at the same oxidation conditions.

Technique of Combined Estimation of the Embrittlement of E110 Zirconium Alloys under High-Temperature Oxidation Conditions

Technique of Combined Estimation of the Embrittlement of E110 Zirconium Alloys under High-Temperature Oxidation Conditions

In-situ micro-Raman study of SnSe single crystals under atmosphere: Effect of laser power and temperature

Single crystal of tin selenide (SnSe) was studied by micro-Raman spectroscopy under atmosphere conditions. The effect of varying the incident laser power on the sample up to 2 mW was analyzed. The Raman spectra showed that the number of all vibrational modes have not decreased or increased, but all peaks red-shifted and softened obviously as the laser power increased to the threshold value. The temperature-dependent micro-Raman study of the single crystal was carried out for illustrating thermal effect due to the high incident laser power. A new SnSe2 phase appeared at high temperature without vacuum and become the dominant phase at the surface of the crystal gradually because of oxidation. Detecting few amounts of SnSe2 crystals on the surface of single crystal shows the high sensitivity of Raman spectroscopy. High resolution transmission electron microscopy (HRTEM) was also used to confirm that the newly generated SnSe2 phase is precipitated by SnSe under high temperature oxidation conditions. To study the Raman spectra of low thermal conductivity materials under high temperature and non-vacuum conditions, lower incident laser power should be used to avoid the influence of additional thermal effects.